By default, the game runs in fullscreen mode, at a resolution of 800x600.

When the user leaves the application while running, for example using the alt-tab keys, the application is reduced to the taskbar.

In this state, when the user hovers with the cursor over the taskbar icon, a thumbnail and preview image of the application should be displayed. That's a feature of the "windows desktop manager", also known as "aero effect". In my case, these preview images are always blank and I couldn't find a good way to fix that. (Please note that the problem is limited entirely to the fullscreen mode - in window mode the preview images work perfectly.)

When the application is moved to the background (while in fullscreen mode), the device object I am using (IDirect3DDevice9) goes into the state of a "lost device" (D3DERR_DEVICELOST). In that state I cannot execute any drawing operation. Only after the application is activated again by the user (returning into the foreground), the application is able to "reset" the device and continue drawing. I suspect this might be the root of the problem: As the application is unable to draw while in the background, the preview images which supposedly reflect the current "screen" state, are also blank.

If that was true, how to fix it? - the behaviour of the device cannot be changed in that regard. I experimented with a mechanism that would automatically switch the application to window mode while in the background. That produced lots of unwanted side effects however, leaving the impression of a bad hack.

I also experimented with the windows desktop manager api. Using it allows for example to set a static bitmap for the thumbnail preview in the taskbar. Unfortunately, the windows desktop manager api is not available on windows XP, so when using and linking it, the application won't run on XP anymore.